Forming Charges, Starved Electrolytes, and Voltage Depression

[radellaf]

I just got my copy of the Battery Handbook in and am having fun reading it... checking the references, though, for the NiMH chapter, the dates are 1994, 97, 98, 96, 99, and 2000. This book badly needs an update given the recent advances in electrode and separator technology I'm seeing in the recent patents.

----

With all the discussions of forming charges and hot spots and such, I thought I'd share (fair use should apply for such a small section) the paragraph that seems to be their take on the whole issue, at least on the NiMH side of things (I have yet to read the NiCd chapter):

"The separator has a crucial role relative to cycle life. In the starved electrolyte design, it is a common design principle to saturate the electrodes with electrolyte at the assembly stage. The separator is designed to have a high electrolyte fill fraction in order to hold as much electrolyte as possible but not be overfilled so as to inhibit gas recombination. To the battery manufacturer, this has the implication that during the first few charge/discharge cycles ("formation") when the electrodes have not yet absorbed all of the intended electrolyte, charging must be initiated carefully to avoid venting."

"...it is common for the separator to be about 90% filled, and then reduced to about 70% during the cell formation process of the first few...cycles as both...electrode(s) expand and contract opening interior regions for electrolyte absorption."

It's not just at formation that happens, either, as the book says that "NiMH cell failure is commonly when the separator fill fraction...10 to 15% of original."

So, you want a separator that holds a lot of electrolyte to start with, and holds on to enough as the cell ages. The reason it moves into the electrodes is capillary action, as the pores in the electrodes are smaller than those in the separator."

----

So, what to make of all that. Sounds like since reduced gas exchange through the separator is the big thing to watch for, which means avoiding overcharge (or discharge) especially much with the new cells.

It doesn't go into such specifics as to say whether overcharge at C/10 for a few hours is at all a problem, but I would assume that limited overcharge at C/3 rates, which normally isn't that bad, would be something to avoid. Even more important would be higher rate overcharge, which is just about guaranteed to happen with 2 hour or less -dV terminated charges.

There's a graph from duracell showing a cell at 1C rate charging with dT/dt termination going for well over 500 cycles, but not for the -dV, saying that the small amount of overcharge is culpable. (note: both lines on the graph are about the same out to around 300-350 cycles, which is probably more than consumer cells get in their lifetime).

So what would I do if I were being exceptionally... fastidious. I guess pull cells early (110% of C put in?) or run C/10 charges for 11-12 hours instead of 16 hours. Dunno if the latter would make any difference, though.

----

So, that's part of the story. If there's any discussion of crystal sizes as relates to separator damage being a reason to run 'forming' charges, it's not in the NiMH chapter.

All they say is "the active materials that have not been cycled change in physical characteristics and increase in resistance."

Oddly, while their graph about this shows about 5% capacity reduction (at 1C to 1V) after 1 month stored charged, the stored discharged graph shows a percent or two _greater_ capacity the first cycle or two out of storage.

 

[SilverFox]

Hello Radellaf,

The "standard charge" is 0.1C for 16 hours. The cells are made so they can handle an over charge at 0.1C without problems, at least for a while.

There was a Swedish study the explored over charging at 0.3C. They found that at this low charge rate over charging the cell resulted in damage to it. Since the probability of missed termination is increased at 0.3C charging, and since over charging at that rate damages cells, it is best to avoid charging at 0.3C.

Duracell tries to buck the system by suggesting that a cell can be fully charged at 0.1C for 12 hours, but when it comes to rating their cells, they follow the standard charge of 0.1C for 16 hours. I would have more respect for their data if they practiced what they preached and rated their cells at 0.1C for 12 hours, but then they would not be confirming to the "standard," so they would probably not qualify for some supply contracts.

The funny thing is that I am not sure that Duracell even makes NiMh batteries. They may just relabel someone elses cells. I put a lot more faith in Sanyo's information. At least I know for sure that they are making their own cells.

In order to figure out how much to charge a cell, you have to figure out the charge efficiency of the cell. At 0.1C and room temperature, the efficiency is around 66%. This varies with charge rate and temperature. This means that you have to put around 150% in to get 100% out. The extra hour during the "standard" charge allows for an even distribution of the electrolyte within the cell.

Theoretically, there is not supposed to be crystal formation during the normal charge/discharge cycle of a NiMh cell. However, impurities and oxidation cause changes that allow these crystals to form. These are formed during over charging, over discharging, self discharge, and extended trickle charging. There is a "plating like" layer of ions that are formed during charging. This layer is reduced during discharging. If this layer is not completely reduced, it changes characteristics and crystals can form. This leads to voltage depression and high internal resistance.

I am not sure if the various battery handbooks go into the forming of the cell, nor the balancing of a multi cell pack. With separator damage being the #1 cause of cell failure, it is prudent to explore everything that can be done to keep the separator healthy.

Tom

 

[LuxLuthor]

Tom, I am beginning to think you know too much about batteries for your own good! :kiss:

 

[wptski]

Tom:

Didn't some manufacturer have cells charging at .1C for over two years without damage?

If I gave you a unmarked cell, how would you check its capacity? I often wonder when a manufacturer came out with a new higher capacity cell, how did they rate it? How do you discharge at .1C when C is unknown????

 

[SilverFox]

Hello Bill,

GP states that their cells can withstand a 0.1C continuous charge for one year. I believe that means that they will still be able to deliver around 60% of their initial capacity at that time, but I am not sure.

When a manufacturer comes out with a new cell, they have a theoretical value that they can start with. This often becomes the labeled capacity of the cell. The minimum value, if stated, is more in line with the tested value.

Let's pick on Sanyo a little. If you look up the data sheet for the 2700 mAh cells, you will find that the actual capacity is more like 2500 mAh. Also, when they give a recommended charge, it is 250 mA for 16 hours. Moving on to the Eneloop cells, if you dig you can find that they consider the actual tested value to be equal to the minimum value.

You can also watch the voltage profile during charging and get a good idea of when a cell is fully charged. Follow this with a discharge and you now have a very close guess of what the actual capacity is.

Tom

 

[wptski]

Tom:

Yes, watching voltage or temperature can be used to tell when a cell is fully charged but the input mAh isn't capacity alone.

 

[radellaf]

...too much about batteries for your own good! :kiss:

Does, doesn't he?

I'm trying to get current on the characteristics of the new improvements, but the data is pretty widely scattered. The handbook is good, but missing a lot of details I've heard from Tom and others. The patents are very interesting, but if I had to guess, I think I'm going to have to start looking up journal articles.

I would like, for instance, to see electron microscope or energy dispersive spectrography of some of these plates and separators. I'm sure a lot is trade secret but there's got to be _some_ public research.

But I've probably learned the most on here. I mean, if I believed half of what's officially published, the Eneloop "Q5" charger wouldn't work. But, it does. Odd how some of the data is so conservative, like the -dV data, and others are so optimistic, like the "2700mAh" ratings

---

As far as capacity, it would be imprecise if you only had time for one charge cycle, but given a few, you can get a good handle on it. Make an estimate, see how that pans out on charge and discharge graphs, refine, repeat.

---

Oh, BTW, I'm going to get to help select a battery for a real project at work. Unlike what probably anyone else at the company would think, I'm thrilled I'm also going to have to edit what I'll want to say about the thing to anyone else here to probably C/10 of what I'd run on at the keyboard about in a forum like this.